Sanitary-ware fixtures, i.e., toilet bowls, lavatories and the like, particularly the former, are constructed of a glazed, highly vitrified, feldspathic body.
The body is usually formed by the well-known method of slip casting, carefully and slowly dried, glazed and fired to vitrification in a tunnel kiln at a temperature equivalent of Seger pyrometric cone 9 to 10. The body is normally referred to as "feldspathic" indicating that a substantial proportion of the mineral content of the slip is feldspar. However, nowadays feldspathic sand or nepheline syenite can be optionally used as a partial or complete replacement for the feldspar without changing this generic description.
Ideally, and as will be described in more detail hereinafter, the casting slip, as well as the final, fired, vitrified body, must meet certain rigid, rheological and physical property standards.
For example, the viscosity, specific gravity and stability of the casting slip should ideally remain constant over appreciable periods of time as these factors are all critical from the standpoint of casting bodies having desired wall thicknesses, drying shrinkage, etc.
The fired body, ideally, will be completely vitrified, have high strength, with minimum, though uniform, shrinkage on firing.
In an effort to either, or both, speed the firing operation and to lower the firing temperature, any number of body additives have been experimented with. For example, many fluxing materials have been tried to speed the body vitrification process, with varying degrees of success. However, such fluxes have generally created more problems than they have solved.
That is, speeding up the vitrification process poses no major problem, in and of itself; but slip instability, excessive shrinkage or warpage on drying or firing, lower strength, short vitrification range, etc., have usually accompanied the utilization of conventional body fluxes used heretofore. By way of explanation, "short vitrification range" refers to the temperature spread, within a commercial kiln, over which the required degree of vitrification will both occur and be sustained without slumping and/or crystallization (devitrification).
Therefore, while a vitrification range would be of little or no significance in a small test bar, fired under carefully controlled laboratory conditions, it is vital to successful manufacture of sanitary-ware products on a commercial scale in a continuous production tunnel kiln. Thus, if a sanitary-ware body reaches vitrification at precisely cone 10 down, while at cone 9 it is not vitrified, whereas at cone 11 it has begun to slump, the production problems become readily apparent because there is no practical way to achieve temperature control at precisely cone 10 throughout the cross section of the firing zone of a production kiln.
A related problem in casting sanitary ware from a ceramic slip in which a glass frit serves as a flux is that the frit tends to be somewhat soluble. Components leached from the frit, notably the alkaline earths, act as flocculents and can sufficiently raise the viscosity of the slip to make it unworkable. In fact, some slips may even virtually solidify. Moreover, this flocculation to an unworkable viscosity can often take place in a relatively short time. Also, sintering of the cast ceramic slip leaves a certain amount of unreacted alkaline earth in the sintered product. This likewise has an undesirable flocculating effect on the slip.
The field of this invention deals primarily with the production of sanitary-ware body additives designed to lower the production firing temperature of a sanitary-ware body, while at the same time preserving all the required characteristics of said body in both the slip stage, as well as in the final, fired product. Further, the invention includes a counterbalancing of flocculents leached by the slip from fluxing frit and a resulting maintenance of a workable viscosity of the slip.
The utilization of either wollastonite (CaO.SiO.sub.2) or diopside (CaO.MgO.2 SiO.sub.2) as an additive for ceramic bodies is disclosed by H. G. Kurczyk in a paper entitled, "Synthetic Diopside and Synthetic Wollastonite--New Raw Materials for Ceramics", from the Proceedings of the 3rd CIMTEC, 3rd International Meeting on Modern Ceramics Technologies at Rimini, Italy, May 27-31, 1976.
Kurczyk discloses the possibility of synthesis of diopside by any number of means, including synthesis by solid-state reaction, crystallization from a molten bath, sintering, etc. Kurczyk's preferred method apparently is the formation of diopside from dolomite and ground quartz in the appropriate molecular ratio by hydrothermal pretreatment and calcination.
However, diopside produced by sintering, from the standpoint of adaptability to ceramic bodies, has a number of drawbacks. First, the sintering process utilizes a rotary kiln operation similar to that used for the manufacture of portland cement clinker. As is well known, it is exceedingly difficult to control the firing temperature in a rotary kiln with any degree of precision, wherein variations in temperature may promote either a "soft burn", resulting in a high percentage of unreacted material, or cause a partially molten bath which solidifies upon discharge from the sintering zone, resulting in the formation of a non-uniform, heavy, slag-type coating on the calciner walls with attendant undesirable interruptions in the overall operation.
Furthermore, the diopside produced by the method disclosed by Kurczyk is relatively large grained (10 to 30 microns) which tends to reduce its effectiveness in lowering the firing temperature of a sanitary-ware body and, as is well known, the diopside mineral crystal tends to be extremely hard and tough, thereby making it most difficult to mill and grind to a particle size small enough to be effectively dispersed throughout a sanitary-ware body slip.
It has been found however that, if the molten bath which Kurczyk found to be undesirable, is deliberately created under controlled conditions, followed by the novel processing steps hereinafter set forth, a more economically produced, superior form of diopside is achieved, which eliminates all the disadvantages of Kurczyk, while providing a number of additional improvements thereover.